Meteorological drivers of compound atmospheric events associated with summertime mortality excess in Spain

Compound climate events involving concurrent heatwaves and air pollution episodes represent a growing threat to public health. This study identifies the large-scale synoptic patterns most frequently associated with such events and their impact on summertime excess mortality in mainland Spain from 2015 to 2022. Using a combination of principal component analysis and cluster techniques applied to ERA5 reanalysis data, we classify 12 circulation types (CTs). Two of them, associated with Iberian thermal lows and upper-level ridges (CT1 and CT4), are linked to simultaneous anomalies in maximum temperature (+4-6 °C), ozone (+10-25μg m^-3) and PM₁₀ (+10-40μg m^-3), together accounting for 25% of summer days. To assess the impacts on mortality, the index M5d is proposed, representing the 5-day forward sum of all-cause deaths. CT1 and CT4 are associated with standardised M5d anomalies exceeding 1.5-2.0 standard deviations in several provinces, indicating a strong health response. Subtypes with Omega block configurations (e.g., CT1.3, CT4.4) show the highest efficiencies, with 70%-80% of days exceeding extreme thresholds for multiple variables and up to 50% efficiency in triggering excess mortality. These findings strengthen the scientific understanding of compound weather-health interactions in Southern Europe.